The present invention relates to an oil-tempered wire, and more specifically an oil-tempered wire having sufficient toughness as a material for high-strength springs used as valve springs for automotive engines.
Valve springs for automotive engines are used in extremely harsh conditions in which they are subjected to high stress and high revolving speed. In particular, valve springs used in recent car engines, which are small in size and consume less fuel, are used in still severer environments. It is therefore desirable to increase the strength of material for such valve springs still further. Valve springs are formed from an oil-tempered wire of chrome-vanadium steel for valve springs or an oil-tempered wire of silicon-chrome steel for valve springs. Efforts are being made to increase the strength of these wire materials.
But a wire having increased strength tends to be low in toughness and ductility, so that it is liable to be broken while being formed into springs.
In order to solve this problem, Examined Japanese Publication 3-6981 proposes to control the content of vanadium and the quenching conditions so that the crystal grain size will be 10 or more, thereby keeping high toughness of the wire. For the same purpose, Unexamined Japanese Patent Publication 3-162550 proposes an oil-tempered wire having a tempered martensite, that is, a matrix after tempering, in which is present a residual austenite phase in an amount of 5-20%.
But in the former, it is impossible to markedly increase the strength and toughness if the crystal grain size is 10 or more. In the latter, if the residual austenite phase is present in a large amount, it may transform into a martensite phase while the wire is used as springs. If this happens, it may suffer a permanent set due to increased volume. That is, such a wire is less resistant to permanent setting.
An object of the present invention is to provide an oil-tempered wire for springs which is less likely to suffer a permanent set and is high in strength and toughness.
As a result of our efforts, we have discovered that it is possible to increase toughness while keeping high resistance to permanent setting by finely dispersing a residual austenite phase in a tempered martensite at a volume rate of 1% to 5% and by controlling the number of carbides having diameters of 0.05 .mu.m or more to 5 or less per .mu.m.sup.2 as observed on a transmission electron microscope (TEM) image.